There is no single “correct” battery size for every UK home. Energy Saving Trust says a typical home battery system might be around 10 kWh, but the best size depends on what you actually want the battery to do: store midday solar for the evening, charge cheaply overnight, provide backup, or a mixture of all three.
The most expensive battery-sizing mistake is to choose capacity from house size alone. A three-bedroom home occupied by two people can use less evening electricity than a two-bedroom home with an EV, electric heating or someone working from home. The useful starting point is your energy profile, not the number of bedrooms.
Battery capacity: what kWh actually means
Battery capacity is measured in kilowatt-hours (kWh). A 10 kWh battery can theoretically store 10 kWh of energy, but the amount you can actually use may be lower depending on the product’s usable capacity, reserve setting and operating limits.
Do not confuse capacity with power. A battery can have plenty of stored energy but still have a discharge power limit that prevents it from covering several high-demand appliances at once.
A better way to size a solar battery
For solar shifting, start with the electricity you regularly use after solar production falls and before the next useful solar window. Then compare that with the amount of solar surplus you are likely to have available to charge the battery.
Practical starting point
Then adjust for usable capacity, efficiency, reserve settings, tariff charging and backup goals.
Daily electricity use is useful — but it is not the battery size
The table below converts annual household electricity consumption into a daily average. It is deliberately not a battery recommendation. Use it as a starting point for deciding how much of that daily use happens outside solar hours.
| Annual electricity use | Average per day | Question to ask next |
|---|---|---|
| 2,000 kWh | 5.5 kWh/day | How much is used after sunset? |
| 2,500 kWh | 6.8 kWh/day | How much solar is exported at midday? |
| 3,400 kWh | 9.3 kWh/day | Would 5–10 kWh cover the useful shift? |
| 4,500 kWh | 12.3 kWh/day | Are heating, cooking or EV loads driving demand? |
| 6,000 kWh | 16.4 kWh/day | Would a larger battery actually refill often enough? |
SolarWorth’s battery tool estimates storage capacity from the variables that actually change battery size, including depth of discharge and efficiency.
Use the Battery Storage Calculator5 kWh vs 10 kWh vs 15 kWh: what changes?
| Nominal battery size | Can make sense when… | Watch out for… |
|---|---|---|
| About 5 kWh | You have modest evening use or only want to shift a few kWh from daytime to night. | It may fill early on sunny days and empty early on high-use evenings. |
| About 10 kWh | You have a larger evening load, more solar surplus or want more tariff flexibility. Energy Saving Trust describes around 10 kWh as a typical home-system size. | Do not assume “typical” means optimal for your home. |
| 15 kWh+ | You have high electricity demand, a larger PV array, time-of-use charging or meaningful backup goals. | An oversized battery can sit partly empty and extend financial payback. |
Five things that should decide your battery size
1. Evening and overnight electricity use
If the battery’s main job is to move solar energy from day to night, this is the most important number. Look at smart-meter data if you have it. A household using 4 kWh between late afternoon and the next morning has a very different storage need from one using 10 kWh.
2. How much solar you export
If your panels rarely create surplus after daytime appliances are covered, a large solar-charged battery may struggle to fill. If you regularly export several kWh at midday, there is more energy available to store instead.
3. Usable capacity and efficiency
A battery’s headline capacity is not always the amount delivered to your appliances. Check usable capacity, depth-of-discharge limits and round-trip efficiency. The SolarWorth calculator lets you include these assumptions instead of treating nominal kWh as fully usable.
4. Charge and discharge power
Capacity tells you how long the battery can supply energy; power tells you what it can run at one time. Kettles, ovens, induction hobs, heat pumps and EV chargers can create high demand. Check the inverter and battery power limits, not just the kWh number.
5. Your goal: savings, backup or both
A battery sized for financial load shifting can be smaller than one intended to provide long backup coverage. If backup matters, decide which circuits are essential and how many hours you want them supported.
Worked example: why two similar homes can need different batteries
Home A
Annual electricity use is 3,200 kWh. Most daytime demand is covered while the panels are generating, and the household uses around 4 kWh from late afternoon to morning. A battery around 5 kWh usable could cover much of that regular shift.
Home B
Annual electricity use is also 3,200 kWh, but the household is empty during the day and uses most electricity in the evening. It exports more solar at midday and uses around 7 kWh after solar production falls. A larger battery may capture more of the surplus — but only if it fills often enough to justify the extra cost.
Same annual electricity use, different energy profile, different battery decision.
What if you have a 4 kW solar system?
“Battery size for a 4 kW solar system” is a common search, but the PV size is only one input. A 4 kWp array on an unshaded south-facing roof can produce much more usable surplus than the same nominal system with shading or high daytime consumption.
Instead of matching battery kWh directly to solar kWp, estimate annual generation, measure or estimate daytime self-consumption, and work out the surplus available to charge the battery.
What if you want to charge the battery from the grid?
Time-of-use tariffs can change the logic. A battery may be used to buy electricity in cheaper periods and avoid importing during expensive periods, even when solar production is low. This can make larger capacity useful — but the economics depend on the tariff spread, battery efficiency, cycle limits and how often the strategy can be repeated.
Do not size a battery without checking ROI
A bigger battery can increase self-consumption, but that does not automatically mean a better investment. Energy Saving Trust says battery storage tends to cost around £5,000 to £8,000. Extra capacity only makes financial sense if the additional kWh are used often enough to create additional savings or tariff value.
After estimating capacity, run the numbers in the Solar Battery ROI Calculator. It separates the question “how much storage could I use?” from “will paying for that storage be worth it?”
Battery-sizing checklist before you buy
- 12 months of electricity use, ideally with smart-meter half-hourly data.
- Evening and overnight consumption in kWh.
- Current or expected solar generation and export surplus.
- Battery nominal and usable capacity.
- Depth of discharge and round-trip efficiency.
- Continuous and peak charge/discharge power.
- Backup reserve you want to keep.
- Battery and inverter warranties, including cycle or throughput limits.
- Total installed price and expected annual financial benefit.
Estimate your battery storage size
Use evening use, backup hours, depth of discharge, efficiency and solar surplus to estimate the storage capacity that fits your goal.
Use the Battery Storage CalculatorSizing and ROI are different questions. Estimate annual battery benefit, payback period and 10-year value before you buy.
Use the Solar Battery ROI CalculatorFrequently asked questions
What size solar battery do I need in the UK?
There is no universal battery size. Energy Saving Trust says a typical home battery system might be around 10 kWh, but the right capacity depends on your evening and overnight electricity use, solar surplus, usable battery capacity, power limits, backup goals and tariff strategy.
Is a 5 kWh battery enough for a house?
It can be enough for a lower-use home or a household that only wants to shift a few kilowatt-hours from daytime solar into the evening. It may be too small for a high-use family, a home with electric heating or a household expecting long backup coverage.
Is a 10 kWh battery enough for a 4 kW solar system?
Possibly, but solar array size alone does not determine battery size. A 4 kW system can have very different surplus depending on location, shading and daytime use. Battery capacity should be compared with the electricity you want to shift into evening or overnight hours.
Should battery size match daily electricity use?
Daily electricity use is a useful starting point, not a rule. Some electricity is used while the solar panels are generating, so the battery normally does not need to store an entire day of consumption unless backup or tariff goals require it.
What is the difference between battery kW and kWh?
kWh is storage capacity: how much energy the battery can hold. kW is power: how quickly it can charge or discharge. A battery can have enough kWh capacity but still be unable to run several high-power appliances at once if its kW output is limited.
Sources and methodology
- Energy Saving Trust — battery storage capacity, sizing factors and typical home systems
- Energy Saving Trust — solar panels and current battery storage cost range
- MCS — UK quality mark for solar PV and battery storage installations
Updated 11 August 2026. Figures are planning examples, not guaranteed outcomes. Solar generation, battery performance, tariffs and savings vary by property, equipment, location and usage. This article is educational and is not financial, legal or engineering advice.